COMMUTATION 



175 



interpole should be 2,720 + 6,080 = 8,800 at full load. The 

 fuM-load current of the machine is 200,000 -f- 440 = 455 amp., 

 and the required number of turns is 8,800 -f- 455 = 19.35. In 

 practice about 21 turns would be put on the interpole of this 

 machine, and if necessary a diverter would be provided to adjust 

 the current in accordance with results obtained on test. 



62. Prevention of Sparking Practical Considerations. It is 

 not suggested that the method of considering commutation phe- 

 nomena as outlined above covers the subject completely. The 

 designer aims at obtaining "ideal" commutation under certain 

 load conditions, knowing well that, even when series-wound corn- 

 mutating poles are used, the required conditions cannot be ex- 

 actly fulfilled at other loads. He relies on the resistance of the 





FIG. 67. Armature coil near end of commutation~period. 



carbon brush to give sparkless commutation even when the con- 

 ditions depart appreciably from those of "ideal" commutation. 

 The extent to which the ideal condition can be departed from 

 without producing destructive sparking is not easily determined 

 except by experimental means. 



In Art. 45, page 146 the effect of the brush-contact resistance 

 was considered, and it was seen that the value of this resistance 

 has no effect on the problem of commutation provided the change 

 of current in the short-circuited coil takes place in accordance 

 with the "ideal" or straight-line law. The reason is that, when 

 "straight-line" commutation is obtained, the distribution of the 

 current over the contact surface of a brush of rectangular section 

 is necessarily uniform. If, now, we wish to examine the condi- 

 tions of commutation when the changes of current do not follow 

 the ideal straight-line law, it is necessary to consider the effect of 

 the brush-contact resistance when the current density is no longer 

 uniform over the entire surface. The diagram, Fig. 67, is gener- 



